Artificial pacemakers have been used for several decades to treat poor heart rhythms. One such arrhythmia is bradycardia. A person with this abnormality suffers from an unacceptably slow heart rate. This condition may be treated by implanting a pacemaker system to artificially increase the heart rate.
The pacemaker system consists of the pacemaker itself and a pacemaker lead. A pacemaker is a small electrical device implanted under the skin, usually below the collar bone. A pacemaker generates electrical impulses at a comfortable, life sustaining rate, 60 beats per minute for example. These impulses are transmitted from the pacemaker to the heart by the pacemaker lead. With the pacemaker output properly adjusted, each pulse causes the heart to beat.
A pacemaker may be constructed or programmed to operate in a variety of different modes. The simplest is the fixed rate mode. A pacemaker in this mode will generate pulses at regular intervals, irrespective of any conditions or responses. A fixed rate pacemaker set at 60 beats per minute will generate one pulse every second. Each beat is conducted along the lead and is transmitted to the heart muscle, also known as the myocardium. The myocardium immediately adjacent the tip of the lead becomes excited, that is it depolarizes. This initiates a series of physiological events that, in total, closely mimic the actions and results of an intrinsically generated heart contraction.
A more sophisticated pacemaker may be programmed to operate in response to the activity of the heart. The pacemaker will only emit a pulse to be conducted to the heart if the heart needs it. In other words, the pacemaker monitors the activity of the heart, and pulses only if the heart rate is too slow. In order to do this, the pacemaker circuitry must be informed of the activity of the heart. The lead performs this function. The lead, thus, performs two important functions in this instance; it allows the myocardial activity to be monitored and it allows pulses to be conducted from the pacemaker to the heart when needed. When a pacemaker is operating in this mode, it is operating in the inhibited mode. It will provide stimulation pulses to the heart at regular intervals unless it is inhibited by ongoing, intrinsic heart contractions.
The pacemaker lead is a flexible, insulated conductor with ends that are particularly suited to the functions they perform. One end must be electrically and mechanically connected to the pacemaker. Accordingly, it has any of a number of configurations that are standard in the pacemaker industry.
At the other end of the lead, the distal end, the lead must contact the heart electrically and mechanically. The manner in which the lead contacts the myocardium may have a profound effect on the efficiency with which the pacemaker pulse is conducted to the heart. A good interface (connection) will allow the heart to be paced with low energy and with little variation over time. This conservation of energy permits longer pacemaker battery life; thereby forestalling a subsequent operation to replace the pacemaker. A bad interface will require larger energy pulses to pace the heart and may, by virtue of the bad contact, cause physical trauma to the myocardium in the region adjacent the tip of the lead.
Past efforts at providing a good interface to the myocardium have resulted in continual improvements. An early standard among pacemaker leads had a smooth, hemispherical, metal electrode at the tip of the lead. This electrode served the dual purpose of pacing and sensing of the heart. At the time of implant the lead was positioned to a location that exhibited good pacing performance (as indicated by low energy depolarization threshold) and good sensing (as confirmed by relatively high voltage sensed signals). Over time, as the heart responded to this foreign body against it, a tissue reaction would ensue. This reaction effectively caused the formation of an unexcitable capsule immediately surrounding the electrode. The pacing threshold (the minimum pacing energy needed to result in a heart contraction) generally rose and then settled to an intermediate plateau; just as the immune system response flared in a large initial response to the foreign body and then subsided to a more stable intermediate level.
Largely through empirical observation, several factors were noted to affect the pacing threshold. Large mechanical stress on the heart resulted in the destruction of tissue adjacent the electrode. This destruction of tissue, known as necrosis, effectively moved the nearest excitable myocardial tissue further away from the electrode. In order to achieve a supra-threshold (successfully depolarizing) pulse, more energy needed to be delivered, since the pulse intensity diminishes roughly with the square of the thickness of the necrotic capsule.
It was noted that for a given lead stiffness, the size of the electrode had an effect on the size of the necrotic capsule that formed around the electrode. A very small electrode was perceived by the heart to be as an arrow; that is, very high stress was induced. A very large electrode was wasteful of the energy it delivered; that is, despite low stress on the heart, it used a great deal of energy because the current needed to be spread over such a large area. An optimum electrode surface area was found to be approximately 9 mm.sup.2.
It was also noted that the surface finish of the electrode had an effect on the threshold. Evidently, a small amount of surface roughness or porosity allowed the body to quickly and efficiently stabilize the electrode with respect to the heart with only a thin necrotic capsule. The in-growth of tissue to the electrode surface apparently minimized shear stress on the heart, and so permitted lower pacing thresholds.
Electrode material was also noted to have effects on the pacing threshold and also on sensing effectiveness. The early pacemaker electrodes displayed a large degree of residual voltage polarization following pacing pulses. Research revealed that some materials displayed better pacing and sensing characteristics.
Other factors have been discovered to improve the effectiveness of pacing and sensing as well as simplifying the placement of leads. Some electrode shapes were shown to create lower thresholds, by creating higher local current intensities for example. Other leads reduced the immune system response to the electrode by eluting an immuno-suppressant at the myocardial interface. Still other leads made lead positioning simpler and more stable by incorporating an anchor by the electrode, typically a tiny corkscrew. Such leads are called active fixation leads.
Nonetheless, there remains a great need to create pacing leads with lower pacing thresholds and good sensing capability. There are several reasons why these improvements are needed. The reasons are basically of two sorts; (1) to improve the reliability and efficiency of state of the art pacing systems and (2) to introduce technology which will allow the state of the art in pacing technology to be extended.
Virtually every pacing system would benefit if pacing thresholds could be reduced without compromising safety or cost. Lowering the pacing threshold would imply that the pacer output could be reduced. This in turn implies longer pacer life and thus more reliability and safety for the patient.
In most patients, the pacing lead or leads are placed and the pacer is programmed so that pacing and sensing are both well performed. In some patients however there are difficulties. Some patients display abnormally high thresholds or loss of capture. For these patients, there is an absolute need for low threshold leads. In other patients sensing is difficult. This is especially true in particular instances. For example, sensing in the atrium (one of the upper chambers of the heart) is almost always more difficult than sensing in the ventricle (one of the larger, more powerful lower chambers). Sensing unipolarly is almost always more difficult than sensing bipolarly (only one electrode in the respective chamber of the heart for unipolar versus two electrodes in the chamber for bipolar). One particularly challenging sensing application is found in single pass leads. Usually, if sensing and/or pacing of both atrium and ventricle is to be done (dual chamber pacing), one lead is placed in the right atrium and a separate lead is placed in the right ventricle. When a single pass lead is employed, only one lead is placed, and it is intended to service both chambers. In this instance, atrial sensing is particularly tenuous.
There is also a need to extend the state of the art in pacing leads. Presently, a single pacing electrode requires so much energy that it is simply impractical to consider placing more than one electrode in one heart chamber. If a pacing lead were available that consumed very little energy it would be practical to place more than one electrode in one chamber. This could result in lead designs that offer unprecedented benefits to the patient. For example, the risk of losing capture due to electrode migration, localized myocardial deterioration (as may be associated with an infarct) or necrosis would be enormously reduced. The availability of multiple pacing electrodes for a single chamber may prove quite effective in the application of anti-tachycardia pacing (pacing the heart in an attempt to interrupt dangerously fast heart rates). The use of multiple electrodes in a single chamber may even permit more physiologic pacing. In other words, it may allow a deployment of electrodes that, when correctly activated, promotes the effectiveness of the heart pumping by increasing the ejection fraction.